The present invention relates to systems and methods for measuring the wavelength and wavelength shifts of electromagnetic radiation, and more specifically to systems and methods for characterizing the wavelength and wavelength shift of a beam of laser radiation.
Lasers such as excimer lasers are employed in a wide range of applications and uses. During many of these applications or uses the output wavelength of the laser must be actively controlled and measured during the operation cycle of the laser. For example, when lasers are used in the fabrication of integrated circuits, the wavelength output by the laser must be well characterized.
Conventional fabrication techniques for integrated circuits use a laser beam to selectively expose an optically sensitive material. Portions of the material not exposed by the laser beam are subsequently etched away. Additional fabrication steps are employed to achieve a circuit matching the shape of the exposed material. The short wavelength of an excimer laser operating in the deep ultraviolet region provides the potential for very sharp edge definition. However, when exposing the optically sensitive material using the laser beam from the excimer laser, the wavelength of the laser beam must be precisely controlled or characterized to ensure that it matches the wavelength for which the optics of the system were designed. A variation in the wavelength of the laser beam can cause the beam to be defocused at the surface of the optically sensitive layer, thereby losing the advantage of the short wavelength and resulting in an imprecise rendering of the integrated circuit. This can adversely affect the electrical characteristics of the resulting integrated circuit, resulting in a poor quality or inoperable circuit.
Accordingly, precise regulation or monitoring of the output wavelength of the excimer laser is important in the fabrication of integrated circuits using excimer lasers. The output of some excimer lasers which are line narrowed can be characterized using an atomic wavelength reference approach described in U.S. Pat. No. 5,450,207, issued Sep. 12, 1995. However, this approach does not work as well for lasers having bandwidths substantially greater than that of the wavelength reference. Therefore what is needed is a system and method to characterize the wavelength of a beam of radiation which has a bandwidth too large to be characterized by traditional means.
The present invention comprises a system and a method useful for characterizing the wavelength and shifts in the wavelength of a tunable excimer laser suitable particularly for partially-line-narrowed systems having bandwidths on the order of 20 to 100 picometers. According to one embodiment of the present invention a beam of radiation output from a laser is sent through a hollow cathode lamp containing a vapor of an absorbing material such as an atom, a molecule, or an ion. The absorbing material has one or more transitions which absorb radiation within the output spectrum of the laser. The transitions in the material in the vapor are fixed in wavelength. Materials can be chosen having transitions at very well known wavelengths which can easily be found in reference books.
After the beam of radiation emerges from the hollow cathode lamp it is then sent into a spectrometer which has a detector array placed at its output. The path taken by the beam of radiation to the detector array defines an optical path for the beam of radiation through the system. The absorption of a portion of the wavelength spectrum of the beam of radiation in the vapor provides a fixed wavelength reference point, referred to as an absorption feature, from which the wavelength or changes in the wavelength of the beam of radiation output by the laser can be measured.
At the output of the spectrometer the wavelength spectrum of the laser beam will be spread out along a dimension of the detector. The wavelength position of the absorption in the vapor is set by physical properties of the transition and will remain fixed even if the wavelength spectrum of the output of the laser shifts. Therefore, as the laser output spectrum varies, the absorption feature due to the transition in the vapor will provide a fixed reference point against which to characterize movements in the laser""s output spectrum.
For a typical partially-line-narrowed excimer laser, the width of the spectrum of the laser will be significantly larger than the width of the transition in the vapor when the two widths are measured by any standard parameter such as full width of the half maximum. The bandwidth of the laser should not be so large as to make the detector impractical.
If two or more absorption transitions at known wavelengths are present in the vapor within the spectrum of the laser output, then an absolute measurement of the wavelength of the laser and the shift in the laser""s output wavelength spectrum is possible. Two absorption transitions with a known separation in wavelength provide a wavelength versus distance calibration for the detector. This calibration can be used to measure the amount by which the output wavelength spectrum of the laser shifts. The two absorption transitions can be from different atomic, molecular or ionic materials in the vapor or in two different vapor chambers, or from spectral shifts in a transition such as an isotope shift. It is noted that the calibration of wavelength versus distance at the detector can also be determined from the physical layout of the system. If this calibration is known then only one absorption line is required to determine the amount by which the output wavelength spectrum of the laser shifts. Another embodiment of the present invention includes a system and method for characterizing the wavelength of a beam of radiation using a known reference transition. The invention comprises a container, a vapor including a material having a transition which absorbs radiation of a known wavelength, the vapor being contained in the container. The container includes an optical path along which the beam of radiation can propagate through the vapor. A dispersive optical element is aligned along the optical path. A detector is aligned along the optical path after the dispersive element.
In still another embodiment of the invention the detector generates an output signal and the output signal includes an absorption feature. The wavelength of the beam of radiation is characterized by comparing the output signal to the absorption feature. In one embodiment of the invention, the output signal includes a first signal corresponding to the spectrum of the beam of radiation absent the absorption feature. The output signal also includes a second signal which corresponds to the spectrum of the beam of radiation after it has passed through the vapor including the material. The second signal contains the absorption feature. The beam of radiation is then characterized by comparing the first signal to the second signal.
In yet another aspect of the invention, the container includes a hollow cathode lamp and the material in a cathode in the hollow cathode lamp includes iron. In another embodiment, the material in the hollow cathode includes platinum. In still another aspect of the invention the beam of radiation has a bandwidth larger than a bandwidth of the transition. The beam of radiation may have a bandwidth for example more than twenty or more than a hundred times larger than a bandwidth of the transition.
In another embodiment of the invention the dispersive element comprises a diffraction grating. In yet another embodiment of the invention, the dispersive element comprises one or more prisms. In still another aspect of the invention, a slit is placed along the optical path so that the beam of radiation encounters the slit before the beam of radiation encounters the dispersive element. In yet another embodiment of the invention, the detector is a photodiode array.
Another embodiment of the invention includes a collimating device placed along the optical path between the dispersive element and the detector. In one aspect of this embodiment, the collimating device includes a lens.
Another embodiment of the invention comprises a laser generating the beam of radiation. In still another aspect of the invention, the laser comprises an excimer laser, and in yet another aspect of the invention the excimer laser comprises a gain medium containing Krypton Fluoride. In another aspect, the excimer laser comprises a gain medium containing Argon Fluoride.